Lunyu II. 17. (33)

There should be no pretense in the profession of knowledge, or the denial of ignorance.
The Master said, "Yû, shall I teach you what knowledge is? When you know a thing, to hold that you know it; and when you do not know a thing, to allow that you do not know it;– this is knowledge."

Legge II.17.

The Master said, 'Yu, shall I tell you what it is to know. To say you know when you know, and to say you do not when you do not, that is knowledge.'